The manufacture of cellulose derivatives in general uses as a starting material a high purity cellulose having an .alpha.-cellulose content in the range of 90-97%. The term .alpha.-cellulose is broadly used to denote the fraction of the dissolving cellulose pulp which is insoluble in aqueous caustic alkali. The soluble fraction, which is called hemicellulose, comprises two main fractions, .beta.- and .gamma.-cellulose. The .beta.-cellulose portion of the hemicellulose fraction may be precipitated by acidification (neutralization of the alkaline hemicellulose-containing solution), whereas the .gamma.-cellulose will remain in solution under such a treatment. In general, hemicellulose consists of heterogeneous polymeric carbohydrates having a degree of polymerization in the range of 25-50.
The first step in the viscose process of making cellulose derivatives is the steeping/mercerizing of the dissolving cellulose pulp. The mercerization step comprises treating the cellulose with a strong alkaline solution usually containing more than 17%. A general account of the preparation of rayon (regenerated cellulose) is set forth at 17 Kirk-Othmer Encyclopedia of Chemical Technology, Rayon, 168-209 (2d ed. 1970); viscose preparation is discussed in detail in pages 175-180.
After the mercerization step the excess steeping liquid is pressed out of the alkalized cellulose, giving a spent liquor which is diluted with respect to sodium hydroxide and enriched with respect to hemicellulose. Since the hemicellulose fraction of the cellulose pulp starting material is dissolved in the strong caustic solution used in the steeping step, a low .alpha.-cellulose content pulp will produce an increased concentration of hemicellulose in the spent mercerization liquid.
After the pulp is converted to alkalized cellulose, it is shredded and aged by exposure to oxygen in the air, resulting in oxidative cleavage of the cellulose molecules to form shorter chains. After a proper period of aging, the alkalized cellulose is brought into contact with a proper amount of carbon disulfide, which converts the alkalized cellulose to cellulose xanthogenate which is soluble in a solution of sodium hydroxide and water.
Some of the spent excess mercerization liquor is normally used for dissolving the cellulose xanthogenate to form viscose, which is then spun into filaments or staple fibers. The major part of the spent liquor, however, is customarily adjusted in volume and sodium hydroxide concentration and is reused for the steeping/mercerization of a new cellulose pulp. In another words, part of the mercerization liquor is recirculated.
After a period of time, the concentration of hemicellulose in the recirculating mercerization liquor will reach an equilibrium which is determined by:
the hemicellulose (.beta.- and .gamma.- cellulose) content in the dissolving pulp;
the hemicellulose content of the alkalized cellulose; and
the hemicellulose content of the spent liquor which is used for the subsequent dissolving of the cellulose xanthogenate.
An excess of hemicellulose in the recirculating mercerization liquor produces a number of disadvantages for the viscose process, which may be reduced or eliminated if the concentration of hemicellulose in the recirculating liquor can be decreased to an acceptable level.
It is well known that a high concentration of hemicellulose in the mercerizing liquor will increase the viscosity of the liquor and thus reduce the rate of penetration of the sodium hydroxide into the cellulose of the pulp, producing uneven mercerization and consequently a lower degree of substitution. This is described in a paper by I. Croon et al. in Svensk Papperstidning 1969 (No. 10) pages 336-38.
Increased viscosity in the mercerization liquor will also produce a lowering of the capacity of the presses used to expel excess liquor from the alkalized cellulose. In order to compensate for the reduced capacity of the presses, it is normal to increase the temperature during the mercerization step, which unfortunately increases the amount of hemicellulose extracted from the pulp. This is described in a paper by H. Bruning et al. in Zellstoff und Papier, 1964/8: 225-32. This paper also indicates that a high hemicellulose concentration in the mercerization liquor increases the solubility of the hemicellulose.
Another disadvantage discovered in the course of my own unpublished investigations with respect to raw pulp consumption is in accordance with these views, namely, that an increased content of hemicellulose in the alkalized cellulose makes it necessary to increase the amount of carbon disulfide used during xanthation in order to achieve the same level of quality viscose as is obtained when using a starting pulp material with a high .alpha.-cellulose content. High strength/modulus fibers cannot be prepared from cellulose materials having .alpha.-cellulose contents in the lower portion of the 90-97% range without simultaneously removing the hemicellulose that is dissolved in the recirculating spent mercerization liquor.
Accordingly, several methods have been devised in the prior art to reduce the hemicellulose content in the recirculated mercerization liquor. The Cerini process, the most well known presently available process, requires dilution of the spent liquor to approximately 6-8% before being subjected to continuous dialysis. As a result, it is necessary to concentrate the liquor by evaporation before it can be reused. At present, only a few viscose producers use this method, as it is relatively costly with respect to maintenance and energy consumption.
According to German Offlegungsschrift No. 24 33 235 and a paper by H. Sihtola et al. at Tappi 57: 73-75 (July 1974), hemicellulose in the recirculated mercerization liquor can be precipitated by the addition of ethanol or methanol. The precipitation agent must have a lower boiling point than that of water, so that the precipitation agent can be recovered by simple distillation. The precipitated mixture is separated by centrifugation or filtration, the centrifugate or filtrate then being distilled in order to recover the precipitating agent. The purified liquor may be reused for mercerization after being adjusted with respect to the sodium hydroxide concentration. This process is complicated and costly both with respect to the capital investment required and to production costs. The use of methanol and ethanol also poses environmental difficulties. So far as I know, this process has not been used on an industrial scale.
Of course, hemicellulose may also be removed by the conventional process of evaporating and reconstituting the mercerization liquor. This common process does not help with the .gamma.-cellulose fraction of the mercerization liquor, however.